This invention relates to apparatus for improving the dewatering capability of a rotary drum vacuum washer or filter. More particularly, dewatering capabilities are improved by the addition of a suction box attachment for increasing locally, with respect to a drum quadrant or section adjacent the suction box, the vacuum force acting upon a substantially insoluble material to be dewatered.
In the prior art, the removal of water from a slurry of a substantially insoluble material comprised of, for example, an inorganic mineral or a cellulosic fiber, using a rotary vacuum filter is well known. Generally, the apparatus comprises a rotary drum with closed ends and open internal structure which is covered with a felt, woven cloth or wire subjected to a vacuum force operating internally upon the drum. A portion of the lateral area of the drum is submerged in a dilute slurry of the material to be dewatered. The vacuum operating upon the filter drum causes a filter cake or web of the material to build up on the submerged portion of the wire or cloth exterior surface of the drum. As the submerged portion of the drum leaves the slurry, the cake is firmly held onto the surface and further water is removed by the action of the vacuum acting upon the exterior cloth of the drum. Wash water may be showered onto the cake or web as the periphery of the rotating drum advances to partially remove soluble contaminants or to displace contaminated solvent from the insoluble material. The vacuum is continuous around the periphery of the drum or at least up until the discharge point is reached, at which point a doctor blade removes the cake or web from the drum's cloth surface.
Many mill installations of rotary filters or washers avoid expensive vacuum pumps by utilizing barometric legs to provide vacuum for dewatering. These devices are somewhat limited in capacity, flexibility of operation and level of vacuum obtainable. As a result of these vacuum limitations and stock washing requirements, most cellulosic pulps, for example, discharge from the rotary stock washers or filters at 10-15% by weight bone dry solids. Generally, where the object of utilizing rotary filters is to wash out contaminants as well as water removal per se, several closely adjacent filters must be operated in line to get adequate displacement of impurities from the insoluble cake material.
Since floor space in most installations is at a premium, an apparatus for improving dewatering which could be attached to existing rotary vacuum drum equipment as installed in the typical dewatering, filtering or washing installation of today without requiring significant layout modifications would offer an attractive economic benefit at low capital cost. Where subsequent drying of the wet product is desired, improved dewatering would mean that less heat would have to be added to obtain a dry product. Where the insoluble material is to be washed free of soluble impurities, improved dewatering would mean that additional showering on existing equipment and better displacement of solution from the filter cake or web would be obtained.
In the past, localized improved dewatering of filter cakes or pulp webs supported upon traveling wires has been obtained by directing the wire over a suction box whereupon the material to be dewatered is subjected to a vacuum force operating upon the suction box. Mazer in U.S. Pat. No. 2,714,839 employes a stationary suction box, subtending an arcuate area of the interior surface of a rotary extractor drum or couch roll over which the wire-supported web passes. A steam hood supplies saturated steam to the wet paper web as it passes over the couch-suction box whereupon the suction box vacuum pulls a substantial amount of steam through the web. At least a portion of the steam condenses in the web, transferring heat to the fibers and its moisture content. Simons et al in an article entitled "Steam Dewatering of Filter Cakes" at 62 Chemical Engineering Progress 75 (January 1976) indicates that part of the dewatering advantage described in Mazer derives from the decrease in filtrate viscosity resulting from the increase in temperature.